Dipeptidyl peptidase IV (DPP-IV, CD26, EC 3.4.14.5) is a serine protease with specificity for cleaving Xaa-Pro and, to a lesser extent, Xaa-Ala dipeptides from the N-termini of polypeptides and proteins. DPP-IV is a non-classical serine protease in that the catalytic triad of Ser-Asp-His, found in the C-terminal region of the enzyme, is in reverse order to that found in classical serine proteases. DPP-IV is widely expressed in mammalian tissue as a type II integral membrane protein. DPP-IV is expressed on the surface of differentiated epithelial cells of the intestine, liver, kidney proximal tubules, prostate, corpus luteum, and on leukocyte subsets such as lymphocytes and macrophages. A soluble form of the enzyme is found in serum that has structure and function identical to the membrane-bound form of the enzyme but lacks the hydrophobic transmembrane domain.
DPP-IV has many physiologically relevant substrates such as chemokines, RANTES (regulated on activation normal T cell expressed and secreted), eotaxin, and macrophage-derived chemokine, neuropeptides such as NPY (neuropeptide Y) and substance P, vasoactive peptides, and incretins such as GLP-1 (glucagon-like peptide-1) and GIP (gastric inhibitory peptide/glucose-dependent insulinotropic polypeptide). GLP-1 is a 30 amino acid peptide hormone produced in the L cells of the distal small intestine in response to ingested nutrients. GLP-1 binding to its receptor on various tissues stimulates insulin gene expression, biosynthesis and glucose-dependent insulin secretion, inhibits glucagon secretion, promotes satiety, slows gastric emptying and promotes growth of pancreatic beta cells. Based on this profile, GLP-l-based therapies are expected to be beneficial in the treatment of type II diabetes and obesity. Studies in which type II diabetic patients have been infused with GLP-1 have demonstrated efficacy in normalizing both fasted and prandial glycemia. However, active GLP-1 (7-36) amide is rapidly converted by DPP-IV to GLP-1 (9-36), which is inactive or is a receptor antagonist. The short half-life of GLP-1 in the circulation (1–1.5 minutes) is a major obstacle to its use as a therapeutic agent. To circumvent the drawback of the short half-life of GLP-1, inhibitors of DPP-IV, the primary degradative enzyme of GLP-1, increase the level of active circulating GLP-1 (7-36) amide. DPP-IV inhibitors have been demonstrated to improve glucose tolerance in type II diabetes.
For a DPP-IV inhibitor to be optimally useful in a human therapeutic setting, it should ideally be delivered as a once-daily oral dose. To accomplish this goal, the compound in question must exhibit both potent inhibition of the enzyme and a desirable pharmacokinetic profile. Because DPP-IV plays a critical role in controlling the degradation of GLP-1, and because the enzyme is ubiquitously expressed at high levels in a variety of tissues as well as within the vasculature, only a potent inhibitor will be capable of impacting circulating GLP-1 levels in a therapeutically relevant manner. In vivo studies suggest that continuous inhibition of DPP-IV leads to a maximal increase in circulating GLP-1, and thus to the greatest improvement in overall glucose control. These results suggest that an inhibitor with a long half-life is most therapeutically desirable.
Several issues complicate the task of preparing a DPP-IV inhibitor that is both highly potent and has an in vivo half-life consistent with once-daily dosing. Many potent inhibitors contain a 2-cyanopyrrolidide functionality in the P1 (catalytic binding site) position. The cyano group of the 2-cyanopyrrolidide forms a covalent linkage with the enzyme through the catalytic Serine that confers increased potency to the inhibitors that contain it by slowing the release of inhibitor.

But the 2-cyanopyrrolidide moiety has liabilities as well. In particular, the cyano-group is found in close proximity to a P2-amine functionality that serves as a marker for the substrate amino-terminus in the DPP-IV inhibitory pharmacophore. When these two groups are held in such proximity, they tend to react to form a cyclic amidine, destroying the pharmacophore. Thus, cyanopyrrolidide-containing DPP-IV inhibitors tend to have limited chemical stability, which is reflected in poor pharmacokinetic profiles.

Compounds of the instant invention are both highly potent and chemically stable, and thus provide unique therapeutic benefit and an improved dosing profile for the treatment of human diseases.